Welcome to the Support Communities!
The article below states that you can browse by title, author, or genre.
When I typed the last group of digits of an ISBN number in the search field, I was able to find the book that way as well.
An Internet connection is required. On iOS devices, iBooks includes the iBookstore. Tap the iBooks icon and your bookshelf appears. In the upper corner, tap the Store button and the bookshelf flips around to reveal the iBookstore. From the iBookstore, you can browse books by title, author, or genre. You can also view featured books as well as books available on the New York Times bestseller lists (United States only). Tap on a book to see more details and download a free sample. When you buy a book, it will automatically appear on your iBooks bookshelf.